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Lewis acid-base interactions in inorganic oxyacids, molten salts and glasses—III
J. inorg, nucl. Chem., 1974, Vol. 36, pp. 43-47. Pergamon Press. Printed in Great Britain.
LEWIS ACID-BASE INTERACTIONS IN INORGANIC OXYACIDS, MOLTEN SALTS AND GLASSES--III CO-ORDINATION STUDIES OF THALLIUM, LEAD AND BISMUTH IN SULFATE/CHLORIDE/BROMIDE GLASS SYSTEMS J. A. DUFFY and M. D. INGRAM Department of Chemistry, The University, Old Aberdeen, Scotland (Received 12 January 1973) Abstract--The behaviour of the basicity indicator ions T1÷, Pb 2 ÷ and Bi 3 ÷ are studied in model glasses having anion compositions: chloride-sulfate, bromide-sulfate and chloride-bromide. Changes in the co-ordination spheres of these metal ions with glass composition are detected spectroscopically from the s-p nephelauxetic effect. When the ligands are chloride and bromide, all three metal ions form mixed coordination spheres, but for chloride-sulfate or bromide-sulfate mixtures, Pb 2÷ and especially Bi 3+ tolerate mixed co-ordination spheres much less readily. Highly polarizing cations in the glasses severely modify competitive ligand behaviour of the constituent anions. The presence of network structures in glasses is shown to give rise to more than one Lewis basicity value, and for measuring basicity it is necessary to know the extent to which the probe ion enters the network. The results also indicate how the modification of an anion's basicity seems to be dependent on the degree of "hardness" of the anion.
IN PART II[1] it was suggested that the differences in Lewis basicity, registered by different probe ions dissolved in glasses and other media, arose as a result of constraints imposed u p o n the probe ions by the structural nature of the media. To understand this effect it is necessary to have a more detailed knowledge of how the probe ions are co-ordinated. Analysis of such coordination spheres in oxyanion glasses is at present impossible, but in glass systems where the composition involves anions of distinct chemical identity, analysis should be more straightforward. This a p p r o a c h is adopted here a n d the "model glasses" contain the anions chloride, bromide a n d sulfate, a n d the highly polarizing Z n 2+ cation. The b e h a v i o u r of T1 +, P b 2+ a n d Bi 3 + in these glasses is observed to investigate how the probing properties of metal ions are sensitive to glass composition. Also, in Part I[2] the setting up of optical basicity scales other than oxide were suggested, and the data for chloride indicated that its basicity is less susceptible to modification by highly polarizing cations than in the case of oxide. In the present study, the basicity of the bromide ion is discussed and related to chloride and oxide in terms of its greater degree of "softness".
bromide-sulfate glasses, the cations again being K + and Zn 2 +. At the all-halide composition the presence of K ÷ ions causes devitrification, and the glasses used in this study were composed of ZnC12 or ZnBr e. It was also found that molten mixtures of ZnCI z and ZnBr z could successfully be quenched to give glasses over the whole composition range, and these were also used. The glasses were made using freshly prepared zinc chloride or bromide (made by passing the hydrogen halide over heated AnalaR zinc and redistilling), and AnalaR ZnSO4, 7H20, KCI and KBr; the ZnSO4, 7HzO was dehydrated before mixing with the other components. The concentration of T1 ÷, Pb z÷ and Bi 3÷ was approximately 10-3-10 -2 M, these ions being added to the glass constituents, as the sulfate or halide, prior to fusion. A small amount of melt was poured on to a silica plate, and covered by a second silica plate, pressure being applied to obtain thicknesses of < 0.5 mm. The resulting sandwich was examined spectroscopically using a Unicam SP700C instrument. The glasses rich in ZnBr 2 were much more prone to devitrification than glasses rich in ZnC12 . Also, very great care had to be taken with ZnCI 2 melts containing Bi3÷, increasing temperature bringing about a darkening of the melt (possibly owing to the formation of lower oxidation states of bismuth [3]). To verify that this did not affect the data for Bi 3 ÷ in ZnC12-ZnBr2 glasses, a parallel set of data was obtained in glasses of composition KzSO4-xZnC12-(IO-x)-ZnBr 2 where this darkening did not occur.
EXPERIMENTAL
RESULTS
As well as the chloride-sulfate glasses described previously[21, it is also possible to prepare the corresponding
The frequencies, v. . . . of the 1S o ---* 3P 1 transition for T1 ÷, P b 2÷ and Bi 3÷ in chloride-sulfate and b r o m i d e -
INTRODUCTION
43
44
J.A. DUFFYand M. D. INGRAM
Table 1. Frequencies, v.... of the IS0 ~ 3P t transition of dl°s 2 probe ions in halide-sulfate glasses. (Frequencies in k K - 1 kK = 1000 cm-1.) Frequencies arising from the tS 0 ~ 1P 1 transition are in parentheses
Vmax(X= CI) Glass composition (mole ratio)
Ratio Ratio X - :SO 2- K + :Zn 2÷
K2SO4-ZnSO4(2:3)
all SO 2- 4:3 1:4 2:1 1:2 22:9 1:1 2:1 2 :1 2:1 3:1 2:1 12 : 1 2 :3 all X all Zn 2 ÷
K2SO4-ZnSO4-KX(7:9:4) K2SO4-ZnSO4-KX(7:9:8) K2SO4-ZnSO4-KX (1:3:4) ZnSO4-KX (1:2)
ZnSO4-ZnX2-KX (4 : 1 : 10) K2SO4-ZnX 2 (1:6) ZnX 2 Halogenocomplex
Vmax(X= Br)
T1÷
Pb 2+
Bi 3+
T1+
Pb 2+
Bi3+
48.0 45.8
46-6 45.5
42-6 42.3
43-2
44.3 43.6 43.2
42'0 41.6 40.6 34.2 33.9
40.6 40-2 40.0
43.0
44.3 43.2 41.3 38.7 38.1
38.6
46.0 * 35.2 34.9 34.0 33.6 32.5
32sh, 42 * 26.8(40)43sh 26.8(39)42sh 26.8(39.0)42.7 27.7 (39) 27.6
40.5
36.8
30.5
38.1
33.2
26.8 (38.5)
* Band envelope revealed no maximum, but indicated the presence of more than one major species. sulfate glasses are in Table 1, and h-function values in these glasses are plotted in Fig. 1 (h' is obtained using: h' = (1 - fls_p)/m, as described in Parts I and II). Since chloride and bromide have much larger h-functions than sulfate (h = 2.0 for CI-, 2.3 for B r - , 1.1 for SO 2in 2K2SO4, 3ZnSO4 glass) the observed decrease in h', with increasing sulfate content of the glass, (Fig. 1) (o) C h l o r i d e - s u l f o i ' e
(b) B r o r n i d e - s u l f o f e
Bi3+ _ ] ~ ~
2"5 2.0 -
"~b
( K>--~ 1,5
"o-
"~-.o I I ,
I~'~F
I°'e-F'e'T--"?
p b z+
2"0 1.5 - ~,,~;~.,~ I.o -I
I
I
I'~
I
I
I
I
20
40
60
80
TI+
1.5 I.O 0
-
20
40
60
80
I00
Anion per c e n t s u l f a f e
Fig. 1. Variation of h-function, as determined by TI+, Pb 2+ and Bi 3+, in (a) chloride-sulfate glasses, and (b) bromidesulfate glasses. (Broken lines indicate regions of composition where more than one major probe ion species exist.)
may be taken to indicate replacement of halide by sulfate in the co-ordination sphere of the probe ion. If the predominating species produced by the probe ions in the glasses have co-ordination spheres radically different from each other (for example if the pure sulfato and pure chloro species coexist as major constituents) then the absorption band envelope should undergo severe broadening, if not splitting to reveal separate peaks. Such behaviour was not apparent for T1 ÷ and Pb 2÷, the band envelope undergoing a gradual blue shift as the sulfate content increased. However, the spectra for Bi 3÷ did not show a smooth, continual blue shift, and it was apparent that even at low percentages of sulfate, e.g. in the K2SO4-ZnC12 (1:6) glass, bismuth(III) complexes of low h' (high sulfate content) and complexes of high h' (high chloride content) were coexisting to a significant degree. This type of behaviour was also apparent in the bromidesulfate glasses, not only for Bi 3÷ but also, to some extent, for Pb 2÷. Figure 2 shows the spectrum of the glass containing 25 per cent sulfate doped with Bi 3 + ; the maxima at 26.8 k K and 39.0 kK are assigned to the tS0 __. apt and 1S0 --* ~P~ transitions respectively of the pure bromo complex, while the maximum at 42.7 kK arises from the ~So --* 3p~ of the pure sulfato species, ( l k K = 1000 cm-1). However, in the ZnC12-ZnBr 2 glasses the shapes of the band envelopes indicated that even for Bi 3 +, co-ordination spheres of mixed chloridebromide constitution are readily produced. For lead(II) in halide-sulfate glasses, it is apparent from Fig. 1 that with increase in the percentage of sulfate, chloride ligands are more readily replaced than bromide ligands; at a halide :sulfate ratio of 1:1 for example, h' has fallen by 0.5 in the chloride-sulfate system but by less than 0.2 in the bromide-sulfate system. Similar behaviour was observed for bismuth(III), where the pure bromo complex would appear to predominate at much higher sulfate percentages than does the pure chloro complex. The greater affinity of bromide for Pb 2+ and for Bi 3+ is also
Lewis acid-base interactions--II
45
glass. It is interesting to note for Bi 3 + that the weak polarizing effect of the Zn 2÷ ions on the bromide ions virtually disappears in the glasses where K ÷ ions are a major cation constituent (Table 1), the 1S0 ~ ap1 frequency of the bromo complex in the glasses having its "unpolarized" value of 26.8 kK. These results are interesting in the light of recent equilibrium studies in alkali metal chloride-aluminium chloride melts, where Lewis acid-base interactions have been correlated with alkali metal size[4]. Since optical basicity is defined as h'/h, it seems that spectroscopic shifts are too small to provide meaningful values in the bromide scale of optical basicity. The range in optical basicity decreases on going from the 0 2- scale to the CI- scale and to the Br- scale, and since anion "hardness" decreases in the order: 0 2- > C1- > Br-, it might be that optical basicity scales have a useful range only when applied to sufficiently "hard" anion systems. Thus, it is anticipated that of the halides, fluoride would provide the widest scale for optical basicity. The weaker Zn-Br interaction explains why the tendency for chloride to displace bromide in the coordination spheres of lead(II) and bismuth(III) (previously found using molten dimethylsulphone as
1.5
I.O
._
"o
e-
0.5
1
I 40,000 Frequency,
I
i
I
30,000
"~ 20,000
c m -I
2 " 2 ~
Bi3+
0
Fig. 2. Optical spectrum of bismuth(III) in high bromidecontaining glass ZnBr 2-KBr-ZnSO 4 (1:10:4 mole ratio). Molarity of bismuth(Ill): 0.02; glass thickness: 0.07 mm. apparent in the data obtained in ZnC12-ZnBr 2 glasses (Fig. 3); for example, in the ZnClz-ZnBr 2 (1:1 mole ratio) glass, the 1So ~ 3P 1 frequency (Table 2) indicates a chloride :bromide ratio of ca. 1:3-5 for co-ordination with lead(II) and ca. 1:6 for co-ordination with bismuth(Ill) (assuming simple additivity of the nephelauxetic effect). In contrast, when thallium(I) was used for measuring the competitive complexation between chloride and bromide, little difference between these two anions was apparent, either in ZnC12-ZnBr 2 glasses (Fig. 3) or in halide-sulfate glasses (Fig. 1).
I
I
I
I
2.4--
pb~+
2.2-
8
0
0
0
2.0
c I
~.e[-
I
I
I
RESULTS AND DISCUSSION The ~S0 ~ 3P 1 frequencies for the three probe ions in ZnCI 2 glass are greater than in their chloro complexes (Table 1). This difference, which can be expressed in terms of h-functions a s (hchloride-h'znCl2) , indicates the fall in Lewis basicity for chloride ions when they exist in ZnC12 glass, and this results from the high degree of donor-acceptor interaction in the C1-Zn bonding. In contrast, bromide ions in ZnBr2 glass (Table 1) lose very little of their Lewis basicity (h'znar2 ~, hbromide): the bromo complexes produced by the probe ions are far less polarized by the Zn 2 ÷ ions than are the corresponding chloro complexes. Thus ZnBr 2 glass might be said to have a "weaker" network structure than ZnC12
2'0
0
18
-~0
1'6
I
I
I
I
40 60 80 mole per ceni" ZnCl2 20
I00
Fig. 3. Variation of h-function with halide composition in ZnC12-ZnBr2 glasses. The straight lines represent values of h' of co-ordination spheres corresponding to the halide mole ratio of the glass composition.
J. A. DUFFYand M. D. |NGRAM
46
Table 2. Frequencies, Vmax, of the 1So -~ aP1 transition of glasses
dl°s 2
probe ions in mixed chloride bromide
ZnCI2 ZnBr2 glasses
ZnC12-ZnBr2-K2SO4 glasses Vmax
ZnBr 2 : Z n C l 2 ratio All ZnBr 2 9 :1 3 :1 1: 1 1 : 1.5 1 :2 1:3 1:5 1:9 1 : 20 All ZnCI 2
Vmax
Tl +
pb 2+
Bi3+
38.6 38-9 40.4 41.1
32.5
27-6
41.6 41.9
33.1 33-8 34.4 34.8 35.8
ZnBr2 :ZnCl2 : K 2 S O 4 ratio 10 :0: 1
27.7
8 :2 : 1 7 :3 : 1 6 :4:1 5 :5 : 1 4:6 : 1 3:7:1 2 :8 : 1
28.3 28.7 28.8 29.1 29.8 30.2 32.3
0 : 10:1
35.4
28-6
32.1 43.0 43.0 43.0
38-1
33.9
solvent[l]) is reversed--compare Fig. 3 with Fig. 1 in Part II[1]. Furthermore, in the halide-sulfate glasses, bromide is again more effective than chloride in displacing sulfate from the co-ordination sphere of Pb 24 and Bi 34 (Fig. 1), and it would appear that the Z n - C I interaction persists in the chloride-sulfate glasses. Preliminary experiments in this laboratory using laser Raman spectroscopy also support this view. The effect of a network structure upon the co-ordinating properties of anions is further apparent in the results of experiments with alkali chloride-borate glasses[5] where it has been found that chloride ions enter the co-ordination sphere of Pb 2 + more easily as the N a 2 0 content of the glass is decreased, that is, as the oxide ions become increasingly involved in the network structure of the glass. The observation for T1 ÷ that the registered h-functions follow closely the "ideal" behaviour represented by the straight line in Fig. 3 possibly shows that T1 + is going into the glass structure in a similar fashion to the K 4 ions, and tends to sense sites dictated by the numerical composition of the medium rather than to seek out one ligand in preference to another. In the ZnC12 glass, optical basicities based upon TI(I) and Bi(III) agree fairly well but are lower than that based upon Pb(II) (Table 3). Pb 2 + appears to be seeking out sites of greater h-function compared with T14 and Bi 3 4. With a network structure, such as exists in ZnC12 glass[6], there is the possibility of a probe ion either entering the network or remaining outside it. Entering the network involves the dl°s 2 ion replacing a Zn 24 cation, and the probe ion is then co-ordinated by chlorides that are attached to only one Zn 2÷ ion. The probe ion will then sense a greater Lewis basicity from these chlorides than will a probe ion that is not incorporated into the network, since the unincorporated probe ion will be interacting with chlorides that are already attached to two Zn 2 + ions. These two types of site are the extremes that the network medium can offer, and it would seem likely that sites of an intermediate nature would also exist. These sites would provide
Bi3+
environments where some of the chlorides are terminal and some are bridging, and the basicity would thus have some intermediate value. F r o m the optical basicity values registered by the probe ions (Table 3), it would seem that Pb 2 + enters the network structure of ZnCI2, but T14 and Bi 34 do not. This seems reasonable in view of the common oxidation number between pb 2 + and Zn 2 4. It is possible that neither TI+ nor Bi ~+ are completely outside the network structure, but are occupying the intermediate type of site described above. A similar choice of sites no doubt exists in sodium borate glasses for which optical basicities using T14 and Pb 2 + were in fair agreement but were greater than those using Bi 34. T14 and Pb 24 would register higher optical basicities if they attached themselves to terminal rather than bridging oxides in the glass. This process involves the metal ions in becoming part of the glass structure. Bi 3+ on the other hand would appear to be excluded from the network and therefore has to attach itself chiefly to bridging oxides, thereby registering a lower optical basicity. Table 3. Optical basicities in ZnC12 glass and ZnBr 2 glass* Scale ATI0) Aeb~ll~ ABi(IlI )
ZnC12 glass 0.83(1) 0"94(6) 0.86(7)
ZnBr2 glass 0.97(1) 1.02(5) or 1.00"I" 0.97(3)
* A values calculated on the basis that the limiting IS0 ~ 3 P 1 frequencies for optical basicities of unity are those for the halogeno complexes (Table 1). t The value of 1.00 is obtained if the limiting frequency is that obtained in molten dimethylsulphone[1]. Acknowledgement--The authors are indebted to Messrs. D. L. Oldfield and A. W. J. M. Rae for much practical assistance.
Lewis acid-base interactions--III REFERENCES
1. J. A. Duffy and M. D. Ingram, J. inorg, nucl. Chem. 36, 39 (1974), [Part ll]. 2. J. A. Duffy and M. D. Ingrain, J. Am. chem. Soc. 93, 6448 (1971).
47
3. N. J. Bjerrum, C. R. Boston and G. Pedro Smith, lnorg. Chem. 6, 1162 (1967). 4. E. Torsi and G. Mamantov, Inorg Chem. 11, 1439 (1972). 5. A. Paul, Phys. Chem. Classes 11, 46 (1970). 6. D. E. Irish and T. F. Young, J. chem. Phys. 43, 1765 (1965).
LEWIS ACID-BASE INTERACTIONS IN INORGANIC OXYACIDS, MOLTEN SALTS AND GLASSES--III CO-ORDINATION STUDIES OF THALLIUM, LEAD AND BISMUTH IN SULFATE/CHLORIDE/BROMIDE GLASS SYSTEMS J. A. DUFFY and M. D. INGRAM Department of Chemistry, The University, Old Aberdeen, Scotland (Received 12 January 1973) Abstract--The behaviour of the basicity indicator ions T1÷, Pb 2 ÷ and Bi 3 ÷ are studied in model glasses having anion compositions: chloride-sulfate, bromide-sulfate and chloride-bromide. Changes in the co-ordination spheres of these metal ions with glass composition are detected spectroscopically from the s-p nephelauxetic effect. When the ligands are chloride and bromide, all three metal ions form mixed coordination spheres, but for chloride-sulfate or bromide-sulfate mixtures, Pb 2÷ and especially Bi 3+ tolerate mixed co-ordination spheres much less readily. Highly polarizing cations in the glasses severely modify competitive ligand behaviour of the constituent anions. The presence of network structures in glasses is shown to give rise to more than one Lewis basicity value, and for measuring basicity it is necessary to know the extent to which the probe ion enters the network. The results also indicate how the modification of an anion's basicity seems to be dependent on the degree of "hardness" of the anion.
IN PART II[1] it was suggested that the differences in Lewis basicity, registered by different probe ions dissolved in glasses and other media, arose as a result of constraints imposed u p o n the probe ions by the structural nature of the media. To understand this effect it is necessary to have a more detailed knowledge of how the probe ions are co-ordinated. Analysis of such coordination spheres in oxyanion glasses is at present impossible, but in glass systems where the composition involves anions of distinct chemical identity, analysis should be more straightforward. This a p p r o a c h is adopted here a n d the "model glasses" contain the anions chloride, bromide a n d sulfate, a n d the highly polarizing Z n 2+ cation. The b e h a v i o u r of T1 +, P b 2+ a n d Bi 3 + in these glasses is observed to investigate how the probing properties of metal ions are sensitive to glass composition. Also, in Part I[2] the setting up of optical basicity scales other than oxide were suggested, and the data for chloride indicated that its basicity is less susceptible to modification by highly polarizing cations than in the case of oxide. In the present study, the basicity of the bromide ion is discussed and related to chloride and oxide in terms of its greater degree of "softness".
bromide-sulfate glasses, the cations again being K + and Zn 2 +. At the all-halide composition the presence of K ÷ ions causes devitrification, and the glasses used in this study were composed of ZnC12 or ZnBr e. It was also found that molten mixtures of ZnCI z and ZnBr z could successfully be quenched to give glasses over the whole composition range, and these were also used. The glasses were made using freshly prepared zinc chloride or bromide (made by passing the hydrogen halide over heated AnalaR zinc and redistilling), and AnalaR ZnSO4, 7H20, KCI and KBr; the ZnSO4, 7HzO was dehydrated before mixing with the other components. The concentration of T1 ÷, Pb z÷ and Bi 3÷ was approximately 10-3-10 -2 M, these ions being added to the glass constituents, as the sulfate or halide, prior to fusion. A small amount of melt was poured on to a silica plate, and covered by a second silica plate, pressure being applied to obtain thicknesses of < 0.5 mm. The resulting sandwich was examined spectroscopically using a Unicam SP700C instrument. The glasses rich in ZnBr 2 were much more prone to devitrification than glasses rich in ZnC12 . Also, very great care had to be taken with ZnCI 2 melts containing Bi3÷, increasing temperature bringing about a darkening of the melt (possibly owing to the formation of lower oxidation states of bismuth [3]). To verify that this did not affect the data for Bi 3 ÷ in ZnC12-ZnBr2 glasses, a parallel set of data was obtained in glasses of composition KzSO4-xZnC12-(IO-x)-ZnBr 2 where this darkening did not occur.
EXPERIMENTAL
RESULTS
As well as the chloride-sulfate glasses described previously[21, it is also possible to prepare the corresponding
The frequencies, v. . . . of the 1S o ---* 3P 1 transition for T1 ÷, P b 2÷ and Bi 3÷ in chloride-sulfate and b r o m i d e -
INTRODUCTION
43
44
J.A. DUFFYand M. D. INGRAM
Table 1. Frequencies, v.... of the IS0 ~ 3P t transition of dl°s 2 probe ions in halide-sulfate glasses. (Frequencies in k K - 1 kK = 1000 cm-1.) Frequencies arising from the tS 0 ~ 1P 1 transition are in parentheses
Vmax(X= CI) Glass composition (mole ratio)
Ratio Ratio X - :SO 2- K + :Zn 2÷
K2SO4-ZnSO4(2:3)
all SO 2- 4:3 1:4 2:1 1:2 22:9 1:1 2:1 2 :1 2:1 3:1 2:1 12 : 1 2 :3 all X all Zn 2 ÷
K2SO4-ZnSO4-KX(7:9:4) K2SO4-ZnSO4-KX(7:9:8) K2SO4-ZnSO4-KX (1:3:4) ZnSO4-KX (1:2)
ZnSO4-ZnX2-KX (4 : 1 : 10) K2SO4-ZnX 2 (1:6) ZnX 2 Halogenocomplex
Vmax(X= Br)
T1÷
Pb 2+
Bi 3+
T1+
Pb 2+
Bi3+
48.0 45.8
46-6 45.5
42-6 42.3
43-2
44.3 43.6 43.2
42'0 41.6 40.6 34.2 33.9
40.6 40-2 40.0
43.0
44.3 43.2 41.3 38.7 38.1
38.6
46.0 * 35.2 34.9 34.0 33.6 32.5
32sh, 42 * 26.8(40)43sh 26.8(39)42sh 26.8(39.0)42.7 27.7 (39) 27.6
40.5
36.8
30.5
38.1
33.2
26.8 (38.5)
* Band envelope revealed no maximum, but indicated the presence of more than one major species. sulfate glasses are in Table 1, and h-function values in these glasses are plotted in Fig. 1 (h' is obtained using: h' = (1 - fls_p)/m, as described in Parts I and II). Since chloride and bromide have much larger h-functions than sulfate (h = 2.0 for CI-, 2.3 for B r - , 1.1 for SO 2in 2K2SO4, 3ZnSO4 glass) the observed decrease in h', with increasing sulfate content of the glass, (Fig. 1) (o) C h l o r i d e - s u l f o i ' e
(b) B r o r n i d e - s u l f o f e
Bi3+ _ ] ~ ~
2"5 2.0 -
"~b
( K>--~ 1,5
"o-
"~-.o I I ,
I~'~F
I°'e-F'e'T--"?
p b z+
2"0 1.5 - ~,,~;~.,~ I.o -I
I
I
I'~
I
I
I
I
20
40
60
80
TI+
1.5 I.O 0
-
20
40
60
80
I00
Anion per c e n t s u l f a f e
Fig. 1. Variation of h-function, as determined by TI+, Pb 2+ and Bi 3+, in (a) chloride-sulfate glasses, and (b) bromidesulfate glasses. (Broken lines indicate regions of composition where more than one major probe ion species exist.)
may be taken to indicate replacement of halide by sulfate in the co-ordination sphere of the probe ion. If the predominating species produced by the probe ions in the glasses have co-ordination spheres radically different from each other (for example if the pure sulfato and pure chloro species coexist as major constituents) then the absorption band envelope should undergo severe broadening, if not splitting to reveal separate peaks. Such behaviour was not apparent for T1 ÷ and Pb 2÷, the band envelope undergoing a gradual blue shift as the sulfate content increased. However, the spectra for Bi 3÷ did not show a smooth, continual blue shift, and it was apparent that even at low percentages of sulfate, e.g. in the K2SO4-ZnC12 (1:6) glass, bismuth(III) complexes of low h' (high sulfate content) and complexes of high h' (high chloride content) were coexisting to a significant degree. This type of behaviour was also apparent in the bromidesulfate glasses, not only for Bi 3÷ but also, to some extent, for Pb 2÷. Figure 2 shows the spectrum of the glass containing 25 per cent sulfate doped with Bi 3 + ; the maxima at 26.8 k K and 39.0 kK are assigned to the tS0 __. apt and 1S0 --* ~P~ transitions respectively of the pure bromo complex, while the maximum at 42.7 kK arises from the ~So --* 3p~ of the pure sulfato species, ( l k K = 1000 cm-1). However, in the ZnC12-ZnBr 2 glasses the shapes of the band envelopes indicated that even for Bi 3 +, co-ordination spheres of mixed chloridebromide constitution are readily produced. For lead(II) in halide-sulfate glasses, it is apparent from Fig. 1 that with increase in the percentage of sulfate, chloride ligands are more readily replaced than bromide ligands; at a halide :sulfate ratio of 1:1 for example, h' has fallen by 0.5 in the chloride-sulfate system but by less than 0.2 in the bromide-sulfate system. Similar behaviour was observed for bismuth(III), where the pure bromo complex would appear to predominate at much higher sulfate percentages than does the pure chloro complex. The greater affinity of bromide for Pb 2+ and for Bi 3+ is also
Lewis acid-base interactions--II
45
glass. It is interesting to note for Bi 3 + that the weak polarizing effect of the Zn 2÷ ions on the bromide ions virtually disappears in the glasses where K ÷ ions are a major cation constituent (Table 1), the 1S0 ~ ap1 frequency of the bromo complex in the glasses having its "unpolarized" value of 26.8 kK. These results are interesting in the light of recent equilibrium studies in alkali metal chloride-aluminium chloride melts, where Lewis acid-base interactions have been correlated with alkali metal size[4]. Since optical basicity is defined as h'/h, it seems that spectroscopic shifts are too small to provide meaningful values in the bromide scale of optical basicity. The range in optical basicity decreases on going from the 0 2- scale to the CI- scale and to the Br- scale, and since anion "hardness" decreases in the order: 0 2- > C1- > Br-, it might be that optical basicity scales have a useful range only when applied to sufficiently "hard" anion systems. Thus, it is anticipated that of the halides, fluoride would provide the widest scale for optical basicity. The weaker Zn-Br interaction explains why the tendency for chloride to displace bromide in the coordination spheres of lead(II) and bismuth(III) (previously found using molten dimethylsulphone as
1.5
I.O
._
"o
e-
0.5
1
I 40,000 Frequency,
I
i
I
30,000
"~ 20,000
c m -I
2 " 2 ~
Bi3+
0
Fig. 2. Optical spectrum of bismuth(III) in high bromidecontaining glass ZnBr 2-KBr-ZnSO 4 (1:10:4 mole ratio). Molarity of bismuth(Ill): 0.02; glass thickness: 0.07 mm. apparent in the data obtained in ZnC12-ZnBr 2 glasses (Fig. 3); for example, in the ZnClz-ZnBr 2 (1:1 mole ratio) glass, the 1So ~ 3P 1 frequency (Table 2) indicates a chloride :bromide ratio of ca. 1:3-5 for co-ordination with lead(II) and ca. 1:6 for co-ordination with bismuth(Ill) (assuming simple additivity of the nephelauxetic effect). In contrast, when thallium(I) was used for measuring the competitive complexation between chloride and bromide, little difference between these two anions was apparent, either in ZnC12-ZnBr 2 glasses (Fig. 3) or in halide-sulfate glasses (Fig. 1).
I
I
I
I
2.4--
pb~+
2.2-
8
0
0
0
2.0
c I
~.e[-
I
I
I
RESULTS AND DISCUSSION The ~S0 ~ 3P 1 frequencies for the three probe ions in ZnCI 2 glass are greater than in their chloro complexes (Table 1). This difference, which can be expressed in terms of h-functions a s (hchloride-h'znCl2) , indicates the fall in Lewis basicity for chloride ions when they exist in ZnC12 glass, and this results from the high degree of donor-acceptor interaction in the C1-Zn bonding. In contrast, bromide ions in ZnBr2 glass (Table 1) lose very little of their Lewis basicity (h'znar2 ~, hbromide): the bromo complexes produced by the probe ions are far less polarized by the Zn 2 ÷ ions than are the corresponding chloro complexes. Thus ZnBr 2 glass might be said to have a "weaker" network structure than ZnC12
2'0
0
18
-~0
1'6
I
I
I
I
40 60 80 mole per ceni" ZnCl2 20
I00
Fig. 3. Variation of h-function with halide composition in ZnC12-ZnBr2 glasses. The straight lines represent values of h' of co-ordination spheres corresponding to the halide mole ratio of the glass composition.
J. A. DUFFYand M. D. |NGRAM
46
Table 2. Frequencies, Vmax, of the 1So -~ aP1 transition of glasses
dl°s 2
probe ions in mixed chloride bromide
ZnCI2 ZnBr2 glasses
ZnC12-ZnBr2-K2SO4 glasses Vmax
ZnBr 2 : Z n C l 2 ratio All ZnBr 2 9 :1 3 :1 1: 1 1 : 1.5 1 :2 1:3 1:5 1:9 1 : 20 All ZnCI 2
Vmax
Tl +
pb 2+
Bi3+
38.6 38-9 40.4 41.1
32.5
27-6
41.6 41.9
33.1 33-8 34.4 34.8 35.8
ZnBr2 :ZnCl2 : K 2 S O 4 ratio 10 :0: 1
27.7
8 :2 : 1 7 :3 : 1 6 :4:1 5 :5 : 1 4:6 : 1 3:7:1 2 :8 : 1
28.3 28.7 28.8 29.1 29.8 30.2 32.3
0 : 10:1
35.4
28-6
32.1 43.0 43.0 43.0
38-1
33.9
solvent[l]) is reversed--compare Fig. 3 with Fig. 1 in Part II[1]. Furthermore, in the halide-sulfate glasses, bromide is again more effective than chloride in displacing sulfate from the co-ordination sphere of Pb 24 and Bi 34 (Fig. 1), and it would appear that the Z n - C I interaction persists in the chloride-sulfate glasses. Preliminary experiments in this laboratory using laser Raman spectroscopy also support this view. The effect of a network structure upon the co-ordinating properties of anions is further apparent in the results of experiments with alkali chloride-borate glasses[5] where it has been found that chloride ions enter the co-ordination sphere of Pb 2 + more easily as the N a 2 0 content of the glass is decreased, that is, as the oxide ions become increasingly involved in the network structure of the glass. The observation for T1 ÷ that the registered h-functions follow closely the "ideal" behaviour represented by the straight line in Fig. 3 possibly shows that T1 + is going into the glass structure in a similar fashion to the K 4 ions, and tends to sense sites dictated by the numerical composition of the medium rather than to seek out one ligand in preference to another. In the ZnC12 glass, optical basicities based upon TI(I) and Bi(III) agree fairly well but are lower than that based upon Pb(II) (Table 3). Pb 2 + appears to be seeking out sites of greater h-function compared with T14 and Bi 3 4. With a network structure, such as exists in ZnC12 glass[6], there is the possibility of a probe ion either entering the network or remaining outside it. Entering the network involves the dl°s 2 ion replacing a Zn 24 cation, and the probe ion is then co-ordinated by chlorides that are attached to only one Zn 2÷ ion. The probe ion will then sense a greater Lewis basicity from these chlorides than will a probe ion that is not incorporated into the network, since the unincorporated probe ion will be interacting with chlorides that are already attached to two Zn 2 + ions. These two types of site are the extremes that the network medium can offer, and it would seem likely that sites of an intermediate nature would also exist. These sites would provide
Bi3+
environments where some of the chlorides are terminal and some are bridging, and the basicity would thus have some intermediate value. F r o m the optical basicity values registered by the probe ions (Table 3), it would seem that Pb 2 + enters the network structure of ZnCI2, but T14 and Bi 34 do not. This seems reasonable in view of the common oxidation number between pb 2 + and Zn 2 4. It is possible that neither TI+ nor Bi ~+ are completely outside the network structure, but are occupying the intermediate type of site described above. A similar choice of sites no doubt exists in sodium borate glasses for which optical basicities using T14 and Pb 2 + were in fair agreement but were greater than those using Bi 34. T14 and Pb 24 would register higher optical basicities if they attached themselves to terminal rather than bridging oxides in the glass. This process involves the metal ions in becoming part of the glass structure. Bi 3+ on the other hand would appear to be excluded from the network and therefore has to attach itself chiefly to bridging oxides, thereby registering a lower optical basicity. Table 3. Optical basicities in ZnC12 glass and ZnBr 2 glass* Scale ATI0) Aeb~ll~ ABi(IlI )
ZnC12 glass 0.83(1) 0"94(6) 0.86(7)
ZnBr2 glass 0.97(1) 1.02(5) or 1.00"I" 0.97(3)
* A values calculated on the basis that the limiting IS0 ~ 3 P 1 frequencies for optical basicities of unity are those for the halogeno complexes (Table 1). t The value of 1.00 is obtained if the limiting frequency is that obtained in molten dimethylsulphone[1]. Acknowledgement--The authors are indebted to Messrs. D. L. Oldfield and A. W. J. M. Rae for much practical assistance.
Lewis acid-base interactions--III REFERENCES
1. J. A. Duffy and M. D. Ingram, J. inorg, nucl. Chem. 36, 39 (1974), [Part ll]. 2. J. A. Duffy and M. D. Ingrain, J. Am. chem. Soc. 93, 6448 (1971).
47
3. N. J. Bjerrum, C. R. Boston and G. Pedro Smith, lnorg. Chem. 6, 1162 (1967). 4. E. Torsi and G. Mamantov, Inorg Chem. 11, 1439 (1972). 5. A. Paul, Phys. Chem. Classes 11, 46 (1970). 6. D. E. Irish and T. F. Young, J. chem. Phys. 43, 1765 (1965).